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The role of neo-tectonics in the sedimentary infilling and geomorphologicalevolution of the Guadalquivir estuary (Gulf of Cadiz, SW Spain) during theHolocene

Antonio Rodrıguez-Ramırez, Enrique Flores-Hurtado, Carmen Contr-eras, Juan J.R. Villarıas-Robles, Gonzalo Jimenez-Moreno, Jose NoelPerez-Asensio, Jose Antonio Lopez-Saez, Sebastian Celestino-Perez, EnriqueCerrillo-Cuenca, Angel Leon

PII: S0169-555X(14)00247-5DOI: doi: 10.1016/j.geomorph.2014.05.004Reference: GEOMOR 4777

To appear in: Geomorphology

Received date: 25 September 2013Revised date: 8 May 2014Accepted date: 11 May 2014

Please cite this article as: Rodrıguez-Ramırez, Antonio, Flores-Hurtado, Enrique, Con-treras, Carmen, Villarıas-Robles, Juan J.R., Jimenez-Moreno, Gonzalo, Perez-Asensio,Jose Noel, Lopez-Saez, Jose Antonio, Celestino-Perez, Sebastian, Cerrillo-Cuenca, En-rique, Leon, Angel, The role of neo-tectonics in the sedimentary infilling and geomor-phological evolution of the Guadalquivir estuary (Gulf of Cadiz, SW Spain) during theHolocene, Geomorphology (2014), doi: 10.1016/j.geomorph.2014.05.004

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The role of neo-tectonics in the sedimentary infilling and

geomorphological evolution of the Guadalquivir estuary (Gulf of

Cadiz, SW Spain) during the Holocene

Antonio Rodríguez-Ramírez a, Enrique Flores-Hurtado

b, Carmen Contreras

a, Juan J. R.

Villarías-Robles c, Gonzalo Jiménez-Moreno

d, José Noel Pérez-Asensio

e, José Antonio

López-Sáez f, Sebastián Celestino-Pérez

g, Enrique Cerrillo-Cuenca

g & Ángel León

h

a Departamento de Geodinámica y Paleontología. Universidad de Huelva. Campus de Excelencia

Internacional del Mar, CEIMAR. Avenida 3 de Marzo, s/n 21007 Huelva (España). arodri@uhu.es

b Espacio Natural de Doñana. Junta de Andalucía. Ctra. El Rocío-Matalascañas, 21760 Almonte (Huelva,

España).

c Instituto de Lengua, Literatura y Antropología. Centro de Ciencias Humanas y Sociales (CCHS).

Consejo Superior de Investigaciones Científicas (CSIC). Calle Albasanz, 26-28. 28037 Madrid (España).

d Departamento de Estratigrafía y Paleontología. Facultad de Ciencias. Universidad de Granada. Avda.

Fuentenueva S/N. 18002 Granada (España).

e Earth and Environmental Science Section. University of Geneva. Rue des Maraîchers 13, 1205, Geneva

(Switzerland).

f Instituto de Historia. Centro de Ciencias Humanas y Sociales (CCHS). Consejo Superior de

Investigaciones Científicas (CSIC). Calle Albasanz, 26-28. 28037 Madrid (España).

g Instituto de Arqueología de Mérida. Consejo Superior de Investigaciones Científicas (CSIC). Plaza de

España 15, 06800, Mérida (España).

h Fundación del Hogar del Empleado (FUHEM), Colegio Lourdes. C/ San Roberto, 8. 28011 Madrid

(España).

Abstract

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A multidisciplinary analysis of cores and geomorphic patterns in the marshes of Doñana

National Park (SW Spain) has yielded new evidence regarding the sedimentary infilling

and geomorphological evolution of the Guadalquivir estuary during the Holocene. The

sedimentation and geomorphological disposition have been strongly conditioned by

neotectonic activity along a set of SW-NE alignments, interrupted by other alignments

that follow E-W and NW-SE directions. The most conspicuous of the SW-NE

alignments is the Torre Carbonero-Marilópez Fault (TCMF). South of this fault, the

estuary experienced a marked subsidence from about 4000 to 2000 cal. yr BP through a

series of sedimentary sequences of retrogradation and aggradation within the context of

relative sea-level rise. From c. 2000 cal. yr BP to the present the subsidence has

remained relatively dormant, with progradation of the littoral systems and infilling of

the marshland progressing within a context of sea-level stability. Our results reveal that

neotectonic activity is a critical factor that must also be reckoned with in any attempt to

understand the Holocene geomorphological evolution in the Guadalquivir estuary.

Key words: Sedimentary infilling, Estuarine facies, Holocene, Neotectonics,

Guadalquivir estuary, South-west Spain.

1. Introduction

It is well known that estuaries evolve as the result of the interaction between

geomorphological structures and dynamic processes that are marine as well as riverine;

this interaction adds up to processes that are inherently estuarine. Local modifications

also intervene; they derive from relative sea-level changes, climate conditions and

human activities (Jackson, 2013). Neotectonic activity (hereafter referred to as

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neotectonics) ought to be included as a factor as well; in combination with the other

parameters, it may cause changes in the sedimentation rates, result in the creation of

new geomorphological features (spits, dunes, cheniers, marshes, levees, alluvial soils),

sea-level oscillations (Kennedy, 2011; Yeager et al., 2012; Feagín et al., 2013) and

associated tsunami and storm inundation (Morton et al., 2007; Nichol et al., 2007). The

subsidence lowers tidal salt-marshes and fertile lowlands below the level of the sea,

which thereafter deposits layers of sediments on the former sub-aerial surfaces until

fresh alluvial sedimentation overrides these layers, only to be buried below sea level in

the next subsidence cycle (Bolt, 2003). Because of relatively rapid developments such

as these, neotectonics must be taken into account in any geomorphic and

sedimentological analysis that aims to reconstruct past environmental evolutions in

littoral areas close to zones of contact between tectonic plates (Rey & Fumanal, 1996;

Ulug et al., 2005).

The Mediterranean basin presents numerous examples of neo-tectonic activity that

conditions the morphodynamic evolution of the coastlines, specifically vertical

displacements related to the collision of the African and Eurasian plates; as in Apulia,

Italy (Mastronuzzi & Sansó, 2012), the south-east of the Aegean Sea (Ulug et al., 2005),

and the Gibraltar Strait (Zazo et al., 1999c). Comparable scenarios have been identified

along the Pacific coasts of North and South America (Atwater et al., 1992; Muhs et al.,

1992; Bolt, 2003; Ota et al., 2004). In North America, for instance, cycles of subsidence

of the coastline have been reconstructed in connection with the punctuated downward

movement of the North American plate vis-à-vis the Pacific plate in the Cascadian

subduction zone (Nelson et al., 2006; Shennan et al., 2006). Across the Western

hemisphere, in the Gulf of Mexico, active fault motion has been suggested as a prime

mover of the recent evolution of the coastline (Yeager et al., 2012; Feagín et al., 2013).

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The multidisciplinary examination of cores extracted from estuaries, lagoons, salt-

marshes, coastal barriers and deltas has revealed a great deal of information about these

evolutions (Vött, 2007; Sorrel et al., 2009).

In the present paper we offer the results of a comprehensive geological study of the

geomorphological patterns recognized in the Guadalquivir estuary, which have been

poorly researched to date. The study included continuous core-drilling and subsequent

lithostratigraphic, paleontological and mineralogical analyses, as well as archaeological

probing and radiocarbon determinations. We contend that sustained neotectonic activity

in the estuary, as in geologically comparable areas elsewhere in the world, has

contributed significantly to the sedimentary infilling and geomorphological

configuration of the basin during the Holocene, marking it out from the other estuaries

in the Gulf of Cadiz. Our results, in effect, indicate that neotectonics has been a critical

factor in the evolution of the entire Atlantic–Mediterranean linkage coast after the

transgressive maximum of the Atlantic Ocean; consequently, it must be so taken into

account in any future understanding of such evolution. We aim primarily at defining a

new palaeogeographic framework in which to ground fresh multidisciplinary research in

an area, such as Doñana National Park and its environs, that is so internationally

attractive from a natural as well as a cultural standpoint.

2. Geological and morphodynamic setting

The Iberian Peninsula lies at the southwest corner of the continental component of the

Eurasian plate, right across the northern boundary of the African plate or Azores-

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Gibraltar fault zone. The present geological structure of the Gulf of Cadiz is the result

of the European–African plate convergence motion, dextral strike-slip along the

Azores–Gibraltar Plate Boundary (Medialdea et al., 2009). This plate convergence

lasted from Mid Oligocene up to Late Miocene times and then continued with slow Late

Miocene to Recent NW convergence (Rosenbaum et al., 2002). Westward drift and

collision of the North African and southern Iberian margins in the Early–Middle

Miocene caused the radial emplacement of huge allochthonous masses (the so-called

“Olistostrome Unit”) in the Guadalquivir Basin (Iberian foreland) and the Gulf of Cadiz

(Torelli et al., 1997; Maldonado et al., 1999). Such emplacement on the Atlantic realm

has been related to the western migration of the Alborán terrain as a consequence of a

once active subduction zone (Royden, 1993; Iribarren et al., 2007). Alternatively,

Gutscher et al. (2002) proposed that this subduction is still active. This complex

geodynamic evolution is recorded in the architecture and tectonic structure of the

continental margin of the Gulf of Cadiz and likely in the sedimentary infilling and

geomorphological evolution of the littoral landforms. In connection with neotectonic

activity in the area, tsunamis—occasionally of rather large magnitude—are a type of

high-energy event that affects the Gulf. There is a large body of data on this type of

event occurring periodically over the past 7000 years (Lario et al., 2011).

The Gulf includes a number of estuaries, all of them partly enclosed by coastal barriers

or spits. Borings taken at some of the largest of these estuaries have provided cores that

are the basis for the current understanding of the sedimentary evolution of this coast of

southwest Iberia during the Late Pleistocene as well as in the Holocene. The most

consistent and reliable results to date have been obtained from borings done in the

estuaries of the Guadalete and Odiel-Tinto rivers (Goy et al., 1996; Dabrio et al., 1999,

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2000; Borrego et al., 1999). Yet the largest estuary in the Gulf, by far, is the estuary of

the Guadalquivir River, enclosed by two spits (Doñana and La Algaida) (see Fig. 1).

The entire area encompasses one of the largest wetlands (50,720ha) in Europe as well as

Doñana National Park, a UNESCOMAB Biosphere Reserve. Paradoxically enough, the

infilling of this large basin in the course of the Holocene has drawn but scant interest.

Significant contributions have only been a fragmentary analysis of a long core (Zazo et

al., 1999a; Lario et al., 2001; Pozo et al., 2010) and research on the period between the

Upper Pliocene and the Quaternary which has failed to concentrate on the evolution

during the Holocene (Salvany et al., 2011).

Paucity of dated cores has combined with a complex tectonic activity in the basin to

render reconstructions of the fossil infill difficult (Zazo et al., 2008; Rodríguez-Ramírez

et al., 2012). Research on the Holocene features has mostly addressed the

geomorphology of the surface formations, although it has resulted in a number of

models of palaeoclimates, palaeoenvironments and sea-level fluctuations charted on a

sound chronological database (Menanteau, 1979; Zazo et al., 1994; Rodríguez-Ramírez

et al., 1996; Rodríguez-Ramírez, 1998; Lario et al., 2001; Ruiz et al., 2004, 2005;

Rodríguez-Ramírez & Yáñez, 2008). The same formations have been pointed out to in

the search for pre-Roman archaeological sites in the estuary and its vicinity (Bonsor,

1922, 1928; Fernández Amador de los Ríos, 1925; Schulten, 1945; Corzo, 1984; Kühne,

2004). The chronological database elucidates a number of periods of climate, natural

environment and sea-level changes, as well as phases of progradation and erosion of

beaches, that unfolded after the transgressive maximum of the Atlantic Ocean c. 6500

14C yr BP (Zazo et al., 1994). As refined from studies of spit systems elsewhere in

southern Iberia, on the Mediterranean as well as on the Atlantic seaboard (Lario et al.,

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1995, 2001; Goy et al., 1996, 2003; Rodríguez-Ramírez et al., 1996; Dabrio et al., 1999;

Ruiz et al., 2004, 2005; Zazo et al., 1994, 2008), the database now includes as many as

six discrete phases of progradation (H1 to H6) following such a transgressive maximum,

each phase separated from the next by an erosive surface or a particularly large swale,

or both, known as a “gap.” Oddly enough, the subaerial record in the spit system of

Doñana—the most extensive in the Gulf of Cadiz, fronting the Guadalquivir estuary—

and that of La Algaida register only part of the H5 phase (the oldest beach ridges

exposed having been dated to c. 2000 cal. yr BP) plus the entire H6 phase, from c. 500

cal. yr BP to the present (Zazo et al., 1994; Rodríguez-Ramírez et al., 1996). Evidence

of phases H1, H2, H3 and H4 is missing. In stark contrast, the Guadalete and Tinto-Odiel

estuaries present exposed prograding units from the H3 phase to the present (Goy et al.,

1996; Dabrio et al., 2000; Zazo et al., 2006, 2008) (Fig. 2).

Behind the Doñana spit system sits a large marshland area (185,000ha)—the heart of

the Guadalquivir estuary—that is the end product of the sedimentary infilling of the

original Holocene basin or palaeoestuary by the Guadalquivir and other rivers, a process

that developed in the manner of a finger-like delta formation extending in a low energy

environment favoured by the growth of the large littoral barriers that isolated the

palaeoestuary from the sea (Rodríguez-Ramírez et al., 1996; Rodríguez-Ramírez, 1998).

With a low gradient, features in the marshland include various muddy morphologies

(levees, channels, point-bars) that have resulted from intense fluvial action, extensive

sandy and shelly ridges (cheniers) resting on the clayey sediments, and marine

processes operating against the barriers (Rodríguez-Ramírez et al., 1996). The wide-

ranging chenier plain formed during two phases of accumulation: from c. 4200 to 2800

cal. yr BP and from c. 1400 to 1000 cal. yr BP (Rodríguez-Ramírez & Yáñez, 2008).

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No surface landforms of the same characteristics dating to a phase in between, of some

1400 to 2800 yr BP, have been encountered (Rodríguez-Ramírez & Yáñez, 2008). The

reason for this anomaly must be found in the tectonic complexity that affects the

Holocene features.

Beneath the marshland sediments, Plio-Quaternary deposits as much as 300m thick

unconformably cover Late Miocene-Early Pliocene blue marls (Salvany & Custodio,

1995) as well as the Olistostrome. As remarked by Armijo et al. (1977) and Viguier

(1977), the geomorphological and stratigraphic architecture of such deposits furnish

evidence of neotectonic activity at least up to the Late Pleistocene along normal faults,

favoring “domino tectonics” that governs the drainage pattern of the rivers and the

coastline as known today. The same authors identified in the structure of the Plio-

Quaternary deposits of the lower Guadalquivir basin two sets of normal faults linked to

an assumed Pliocene extensional phase: a main set of N–S oriented faults that includes

the lower Guadalquivir and Guadiamar-Matalascañas faults (BGF and GMF,

respectively) and a subordinate system of E–W oriented faults that mainly developed in

the westernmost part of the basin (see Fig. 1). Later studies by Goy et al. (1994, 1996),

Salvany & Custodio (1995) and Zazo et al. (2005) have described comparable E-W

oriented faults—such as those of Torre del Loro (TLF) and Hato Blanco (HBF)—that

shape the more recent sedimentation in the Huelva coast during the Pleistocene (Fig. 1).

Furthermore, studies by Flores-Hurtado (1993) and Salvany (2004) have identified NW-

SE and NE-SW oriented systems of alignments and a regional tilting of the basin

toward the SSE as the dominant structural features that impinge upon the sedimentation.

The submerged shelf of the Gulf of Cadiz is also criss-crossed by a system of

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alignments, most of which are NE–SW and NW–SE oriented; these alignments nurture

mud volcanoes and diapiric processes (Medialdea et al., 2009).

3. Materials and methods

3.1. Geomorphology

We analyzed by photo-interpretation aerial photographs taken in 1956 (1:33,000), 1998

(1:10,000), and 2010 (1:10,000) in order to define the geomorphology of the

sedimentary bodies, including the anomalies in the geomorphology caused by the rivers

and the vegetation. The initial cartography of the resultant morphologies was

complemented by direct observations in the field.

The Topographic Map of Andalusia (1:10,000) served as a base document for mapping.

More detailed topographic information, obtained by Light Detection and Ranging

(LiDAR) surveying, made it possible to inspect at closer, centimetre-scale the

formations in the marshland. This information was checked with the said series of

aerial photographs by using a Geographic Information Systems program, gvSIG.

3.2. Lithostratigraphy

The investigation for this paper concerns the sedimentary sequence registered in 5

cores, of depths ranging from 4 to 18m (MA, S6, S8, S9 and S11 (Fig. 1)), all but one of

which obtained between 2006 and 2010. We also examined the uppermost section—

pertaining to the Holocene formations—of 5 other cores (ML, PN, TC, CM2 and CM3

(Fig. 1)) that the Geological and Mining Institute of Spain (IGME) had drilled in the

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area previously; the borings were taken mainly by a direct circulation rotary method,

with continuous core sampling (Cruse, 1979) (see Table 1). In addition, we relied on a

number of shallow (< 3 m) test trenches and pits (CV, LSM (Fig. 1)) dug in surface

formations by means of a 2cm diameter Eijkelkamp gouge and an 8cm diameter

helicoidal drill.

The aim was to identify the morphology, geometry and extension of the sedimentary

bodies, as well as the relationship they bear with underlying and overlying sediments.

The stratigraphy and lithology of each core was described during field sampling. Grain-

size distributions were determined by wet sieving for the coarser fractions (>100mm);

for fractions lesser than 100mm, grain-size analysis required photosedimentation on a

Mastersizer 2000 laser diffractometer (“Sedigraph 5100”).

3.3. Palaeontology

A general revision of the taxonomical determination as well as the estimation of

densities and diversities of species (faunal composition and shell taphonomy) was

possible by means of a macrofossil analysis. Samples of the formations were collected

and prepared by washing the bulk sediment (12 cm3) through a 1mm sieve. Bivalves

and gastropods were identified to the species level and then counted to determine the

semi-quantitative distribution of the species in each core. The presence and relative

abundance of other groups (scaphopods, barnacles, bryozoans, etc.) were also noted.

3.4 Mineralogy

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Mineralogical analyses were conducted by powder X-ray diffraction (XRD) techniques

on a Bruker-AXS D8-Advance diffractometer, using monochromatic CuKα radiation at

40kV and 30mA. Random powders were scanned from 3 to 65° 2θ at 2° 2θ/min, after

gentle grinding and homogenisation to <63μm.

3.5. Dating

Twenty radiocarbon dates were considered relevant to this study. Ten of them apply to

samples from mollusc shells which were subjected to the AMS method in the

laboratories of Beta Analytic (Miami, USA), Centro Nacional de Aceleradores (Seville,

Spain), and Accium BioSciences Accelerator Mass Spectrometry Lab (Seattle, USA).

The shells selected were those that showed, wherever possible, a low degree of transport

or no transportation, and were preserved as articulated valves in the sedimentary record.

The remaining ten dates had been obtained by other authors (see Table 2). Calibration

relied on version 6.0 of the CALIB program (Stuiver & Reimer, 1993) as well as on the

calibration dataset of Stuiver et al. (1998). Correction of the reservoir effect followed

A. M. M. Soares’s guidelines regarding calibration of marine shells from the Gulf of

Cadiz (Soares & Martins, 2010). Expressed in Delta-R (ΔR) values, conditions of the

reservoir effect are known to vary over time as well as space. For the Late Holocene on

the Andalusian coast of the Gulf of Cadiz, Soares recommended a ΔR value of −135±20

14C yr; except for the range 4400-4000

14C yr BP, for which he suggested a value of

+100±100 14

C yr (see Soares & Martins, 2010, for details). Lack of sufficient data for

the 4,000 to 2,000 14

C yr BP preclude determining with enough assurance the most

recent time boundary to which the +100±100 ΔR value can be extended. We chose to

extend it to the middle 14

C year 3000 BP. New data on reservoir effects are necessary to

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calibrate with greater precision samples from the Gulf of Cadiz. Final date estimates in

the dataset specify 2σ intervals (see Table 2).

The resultant isotopic determinations were compared with the age of archaeological

remains known from the area (Bonsor, 1928; Corzo, 1984; Campos et al., 1993, 2002).

In addition, pottery shards were discovered in and extracted from the littoral strands of

Carrizosa-Vetalarena; they were sent to Instituto de Arqueología del CSIC (in Mérida,

Spain) for typological analysis and dating.

4. Results

4.1. Geomorphology

A detailed cartography highlighting the surface features (spits, dune systems, cheniers,

beach ridges, levees and present-day hydrography) reveals a number of clearly distinct

geomorphological units (Fig. 1), as described in the following sections.

4.1.1. The Abalario Dune Systems

This remarkable unit, north of the research area, comprises different semi-stable dune

systems that move in NW and W directions. The dunes can be as high as 70m above

sea level. Marine erosion results in cliffs that are 10 to 15m high. Altitudes decrease

according to a north to south pattern. South and east, the dune formations gradually

fossilize below the present-day marshes and the Doñana spit. Lagoon environments, in

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association with dune fronts and deflationary basins, have developed in between,

generating notable wetlands.

4.1.2. Dunes in the spits

Overlying the Abalario unit and the marshland, this spectacular complex includes two

systems already mentioned: Doñana and La Algaida. The two systems are connected to

the prograding phases of the coast. The most recent dunes (conspicuous in the Doñana

spit) have developed from being foredunes to becoming well-formed transversal bodies,

as much as 30m high. Below this transversal system lies a second one toward the inner

side of the estuary; the dunes here are lower—no more than 10m high—and have

evolved into parabolic dunes. This lower system constitutes the central area of the La

Algaida spit.

4.1.3. Littoral strands in the spits

Identified and mapped in the southernmost—i.e., most recent—sector of the Doñana

spit, these successive, overlapping landforms signify an intense progradation of the

coast. The strands, 4 to 7m high, have accumulated in a S-SE direction; some are

covered by foredunes. Out of a total of 21 strands, the first six curve toward the west,

while the rest, beginning from an erosive line, do so toward the east. Although the spit

shows a marked alternation between ridges and swales overall, the dunes covering the

most recent strands hide this alternation. On the La Algaida spit, the littoral strand

surrounds the central area and connects with the mainland in the form of a tombolo.

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4.1.4. Marshland levees

Part of a deltaic system that has been filling the estuary, these clay-rich levees flanking

the Guadalquivir River and its relict beds have a variable width (300 to 2,000m) and a

great length. They can reach heights of 0.5 to 1.8m above the adjacent inter-levee

marshes or flooded areas. Altitude diminishes progressively from north—2m—to

south—0.5 to 1m near the mouth of the river—on a slope with a gradient of 0.004%.

As we shall see, two different sectors can be distinguished, each with a distinct

configuration.

4.1.5. Cheniers

Sandy and shelly deposits with a littoral strand morphology overlie the clayey infilling

of the marshland, their original shape having been blurred by the activity of the fluvial

network. The sandy formations of Carrizosa-Vetalarena and Vetalengua are distally

attached to the Doñana spit and oriented toward the NE over long distances; in the case

of Carrizosa-Vetalarena, as much as 10km (Fig. 1). These formations, 50 to 100m wide

and 2.15 to 2.25m high above sea level, consist of overlapped strands. The cheniers are

deposits of abundant, almost exclusive mollusc shells, chiefly shells of Cerastoderma

edule; those of Marilópez and Las Nuevas stand on the south-west banks of the

marshland levees and on sandy formations. Their morphology is that of beach

formations, with narrow, small ridges and landward-dipping crests. At points S6 and

PN, the sandy and shelly deposits found in the uppermost section of the cores belong in

the littoral strand system of Carrizosa-Vetalarena and the chenier of Las Nuevas,

respectively.

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4.2. Geomorphological anomalies.

Detailed analysis of the aerial photographs and satellite images, LiDAR information and

topographic maps enabled us to identify a number of geomorphological anomalies, such

as peculiar drainages, subdued morphologies, erosive forms, even patches of vegetation

caused by weaknesses and changes of humidity in the ground.

Remarkably orthogonal, rectilinear alignments marked by the hydrographical network

stand out in SW-NE (≈N50-60E) and NW-SE (≈N110-130E) directions (Fig. 3). These

directions, which define the northern and southern boundaries of the marshland area,

match the directions of the Neogene formations located farther north (Fig. 1). Two

topographical sections (C-D and A-B) reveal sharp level differences of 10 to 30cm (Fig.

4). Seemingly not much of a topographic gradient in such an extensive area, the

abruptness of the offset makes it significant.

The erosive inflection of El Puntal also calls the observer’s attention, as does its

projection along the Carrizosa-Vetalarena sandy ridge in the same SW-NE direction

(Figs. 1, 3). Such alignment cuts through the right-hand levee of the Guadiamar

channel, leaving toward the north a surface of old marshland—rather degraded at some

locations (Fig. 5)—which rises 1.80m over the surrounding plain. South of the

alignment, the levees show a different arrangement: much more developed

longitudinally than transversally. In addition, there are remnants of block-like surfaces

that exhibit an odd drainage pattern (Fig. 3): a sign of probable tilting in a southerly

direction.

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4.3. Lithostratigraphic, paleontological and mineralogical analyses of the cores

The cores exposed a series of Holocene deposits (in marshland, dunes and beaches)

resting on pre-Holocene formations of sands and clays that indicate dunes and marshes

(Fig. 6 and Table 3):

4.3.1. Pre-Holocene Marshland Formation (PHMF)

A substratum for the subsequent Holocene deposits, this facies of grayish ochre

(10YR8/2) silts turned up in cores S11, ML, and PN at depths of -13, -11 and -28m,

respectively. The silts make up 0-75%; clay, 20-25%; sand, 2-5%. Burrowing, roots,

carbonate nodules, oxidation, and intense lamination were found. Macrofauna were

absent. Detrital minerals (quartz, feldspar) amounted to 6-10% and 2-4%, respectively.

Calcite (15-35%), filosilicates (50-70%), and traces of dolomite (7%) were also

encountered.

4.3.2. Abalario Dune Formations (ADF)

White-orange (10YR8/6), reddish-tinted (5YR6/8) sands, lightly crusted at some points,

appeared in cores TC, S6, S9 and MA at -19, -11, -7 and 0m, respectively. At the MA

point, the top of the dune formation is present at sea level, surfacing as a result of wave-

induced coastal erosion and becoming fossilized under the current beach deposits and

foredunes. The plane of contact between the ADF formations and the subsequent

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Holocene deposits leans progressively toward the south, down to -19m at the TC

location.

Lithostratigraphic analysis revealed 75-90% sand, 5-20% silt and 1-4% clay. The sand

is made of well rounded, well sorted grains, 65% of which measure from 250 to

1000µm. Mineralogical analysis yielded 75-95% quartz, 5-14% feldspar, 8-10%

filosilicates and 2-3% dolomite. No remains of macrofauna were encountered. On a

regional scale, the sequence has a thickness of as much as 100m (Salvany et al., 2011).

North of the study area, marine erosion of the formation has resulted in a cliff 15 to 20m

high, where one can identify several aeolian sequences that contain traces of dune slacks

in the form of peats. These peats have been dated to the Upper Pleistocene (Zazo et al.,

1999b, 2008).

4.3.3. Holocene Estuarine Formation (HEF)

The thickness of these deposits turned out to vary greatly according to the location

chosen for the drilling. At point S9 the thickness is some 6 m below ground; at S6, S8,

S11 and ML, it is between 11 and 13m; at PN, as much as 28 m. The ground surface at

all these points stands roughly 1 to 2.5m above sea level. At CM3 and CM2, where the

sedimentary record is more discontinuous, the ground surface is 6m above sea level

(Table 3).

In the inner, central part of the estuary, the prevalent lithology is one of grey (5Y 8/2)

clayey silts (Table 3), with some sand: 65-75% silt, 20-30% clay, 0-6% sand. The sand

decreases gradually toward the top of the sedimentary sequence. Macrofauna are

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present: mostly Tellina tenuis, Cerastoderma edule and Crassostrea angulata, the

remains decreasing toward the top. The top 1m registered the prevalence of terrestrial,

freshwater species (Cochlicella, Helix, Melanopsis). Burrowing and lamination were

also found, with stems and roots in the top. The mineralogical analysis revealed an

abundant presence of filosilicates (60-80%) and lesser amounts of quartz (10-30%),

calcite (5-20%), feldspar (3-10%) and dolomite (2-5%).

In outer sub-areas of the estuary, such as the Doñana spit, cores CM3 and CM2 yielded

successive deposits of estuarine facies separated from one another by intervals of the

sandy facies of the HSF, described below (Fig. 6). Because of the greater marine

influence in the Doñana spit, at CM3 and CM2 such sandy facies contained sandy silts

made of grains coarser (10-30% fine sand, 30-40% silt, 10-15% clay) than those found

in the cores from the central part of the estuary (Table 3). The intervals—consisting of

quartz (10-20%), filosilicates (30-50%) and calcite (15-20%)—had burrows, roots and

lamination, as well as remains of transported marine malacofauna (Glycimeris sp.,

Ostrea sp.) and estuarine species (Cerastoderma edule, Tellina tenuis) showing a low

degree of transport or no transport

Interspersed with the silt sequences were also facies of sandy layers (40-70% sand, 25-

40% silt, 5-10% clay) (Table 3) that resulted from erosive interruptions in the

sedimentary build-up of the estuary caused by high energy events such as storm surges

or tsunamis. The thickness of these sandy facies ranges from a few decimeters at some

locations to over 1m at others. The mineralogical composition is largely quartz (50-

95%), complemented with filosilicates (10-40%), feldspar (10%) and dolomite (10-

15%). Palaeontological remains concern species as highly diverse as Chlamys

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multistriata, Chlamys flexuosa, Venus sp., Glycymeris sp., Chamelea gallina, Murex

brandaris, Solen marginatus, Cerastoderma edule, Crassostrea angulata and Tellina sp.

One of these marine erosion-related, sandy deposits, of varying depth—and turning into

silt sediments toward the top—marks the entire HEF sedimentation from the facies that

underlie it (ADF and PHMF). At S9 these deposits clearly indicate an erosive event as

they contain a massive accumulation of shells (both articulated bivalves and

disarticulated valves) and shell fragments in a sandy-muddy matrix with gravel and

lithoclasts on an erosive base on top of ADF. At S6 this facies constitutes the top of the

core, which corresponds to the Carrizosa-Vetalarena littoral strand, mentioned above,

extending over many kilometres farther inland through marshland (Fig. 1). At point

CV, farther south, a different, more recent sandy facies makes up Vetalengua, the other

littoral strand system that is part of the Doñana chenier plain.

Other sediments in the HEF consist of layers of bioclasts (Table 3) of malacofauna

(mostly Cerastoderma edule and Crassostrea angulata) in a grey (5Y 8/2) silt-clayish

matrix containing some sand (40-60% silt, 25-30% clay, 10-30% sand). The mineral

content consists of filosilicates (50-70%) with lesser amounts of quartz (10-20%) and

calcite (15-25%). At PNL the very top of the core turned out to have a littoral strand

morphology that also spreads at length over the marshland area, as part of the large

chenier of Las Nuevas.

4.3.4. Holocene Spit Formations (HSF)

These facies are clearly represented at points TC, CM2 and CM3 by siliceous sands that

indicate dune events and beaches (Table 3).

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At TC, CM2 and CM3, sands documenting dune accretion turned up in the top 2.5, 3

and 3.5m, respectively, of the core, corresponding to the present foredunes. At CM3,

the two sand layers—found from the top down to -8m and from -14m to -18m—exhibit

a composition that is homologous to that of the present dunes. They are yellow

(10Y8/6) fine-to-medium-grained sands, very well sorted and rounded. As much as

55% of the sediment exhibits a mean grain size of 125 to 500µm. The sediment

includes mostly quartz (70-85%) with filosilicates (15-25%) and feldspars (5-10%);

with no macrofossils.

Sands indicating beaches are ochre-yellowish (2.5 7/6), fine-to-coarse-grained sands,

poorly sorted (Table 3). The TC core yielded a continuous record with coarser deposits,

including coarse pebbles. At CM2 these sand intervals appeared in succession

throughout the core, each interval separated from the next by an estuarine facies (Fig.

6). Calcite (10-15%), filosilicates (10-20%), feldspars (15-25%) and especially quartz

(40-55%) made up the mineralogical composition. The sands contained paleontological

remains. A wide diversity of species of malacofauna, rather deteriorated by a high

degree of transportation, was identified: Chlamys multistriata, Cymbium olla, Panopea

sp., Chlamys flexuosa, Venus sp., Glycymeris sp., Murex brandaris, Venus sp., Tapes

sp., Chamelea gallina and Solen marginatus. The record at LSM, on a littoral strand of

the Doñana spit, corresponds to such facies in the top of the CM2 core.

4.4. Chronology

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As noted above, twenty 14

C dates were considered for the study, ten of which we

ourselves had obtained in the course of the investigation (Table 2). The oldest

formation detected in the cores was the Pre-Holocene Marshland Formation (PHMF),

with radiocarbon ages ranging from 19,000 BP to > 47,000 BP. Point S11 yielded a

radiocarbon age of 19,360±80 BP at a depth of -13m, the top there of the PHMF. Point

PN produced an age of >46,000 BP at a depth of -35.5m; point ML, an age of >47,000

BP at -27m. All of these radiocarbon determinations could not be calibrated, as they are

out of range for the calibration.

The remaining radiocarbon determinations, from c. 8000 cal. yr BP up to the present,

pertain to facies of the Holocene Spit Formations (HSF) and the Holocene Estuarine

Formation (HEF). In all the cores analyzed, formations HSF and HEF appeared

consistently overlying both the PHMF and the Abalario Dune formations (ADF);

furthermore, in all the cores drilled in the inner estuary (S6, S8, S9, PN and ML) the

HEF appeared to start with a high-energy sand deposit containing abundant marine

fauna, from which we collected most samples for 14

C dating.

Point PN yielded the oldest radiocarbon determination for the HEF: 6543-6789 cal. yr

BP. In contrast, points S11, S6 and ML place the earliest Holocene sedimentation in the

years 5255-5461 cal. yr BP, 5693-5910 cal. yr BP and 5728-6012 cal. yr BP,

respectively. At S9 the date obtained for the same facies is more recent: 3830-4410 cal.

yr BP. At TC the Holocene Spit Formations (HSF) produced as early a date as 7793-

8116 cal. yr BP.

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The emerged formations with the oldest 14

C dates are the littoral strands of Carrizosa-

Vetalarena (at point S6), 4071-4706 cal. yr BP, and Vetalengua (at point CV), to the

south of the former, 1825-2149 cal. yr BP. As remarked above, both Carrizosa-

Vetalarena and Vetalengua are part of a chenier plain system formed within the estuary.

As to the perimeter of the Doñana spit, the oldest littoral strands known (at point LSM)

produced a date as recent as 1883-2152 cal. yr BP; there, strands are the same as those

that cap the sedimentary record at CM2.

The 14

C dates obtained were in accord with the age of archaeological remains

encountered in the area. The top of CM3 is closely related to remains from the 2nd to

the 6th century CE (Bonsor, 1928; Campos et al., 2002). The pottery shards extracted

from the littoral strands of Carrizosa-Vetalarena date from the late Neolithic and Copper

Age in south-west Spain (roughly 5,500 to 4,000 cal. yr BP). Neolithic and Copper Age

finds are also known from large tracts north and north-west of such strands, within the

present-day geomorphological area of El Abalario (Campos et al., 1993; Campos &

Gómez-Toscano, 1997).

5. Discussion

5.1. Pre-Holocene formations

The ten cores selected register a pre-Holocene substratum of qualitatively distinct

material (PHMF and ADF formations) at a varying depth. In the cores TC, S6, S9 and

MA, it is the ADF (Abalario Dune Formations) that constitutes this substratum. The

different depths at which these dune formations appear—-19m, -11m, -7m and 0m,

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respectively—indicate a progressive, non-synchronous sinking of the ADF toward the

south.

To the north, within the present-day geomorphological area of El Abalario (Fig. 1), the

ADF has been subject to especially destructive coastal erosion, the effect of which being

the generation of a large cliff there. By studying the position of a number of 16th

- and

17th

-century watchtowers in the area vis-à-vis the shoreline, a rate of cliff recession of

0.7 to 0.8m/yr on average has been calculated for the La Higuera watchtower, near the

MA location (Fig. 1). To get some idea of the approximate coastal recession accrued

there over longer time spans, one can apply such rate to the past 2500 and 5000 years,

obtaining as a result that the shoreline may have receded as much as 1800 m and 3600

m, respectively, since the mid Holocene.

Laterally toward the centre of the estuary, the ADF seems to interdigitate somehow—no

reliable data are available to describe it—with the PHMF formation, as the latter

appears at points S11, ML and PN at depths of -13, -11 and -28m, respectively. The

presence in the PHMF deposits at these locations of burrowing, roots, carbonate nodules

and intense lamination suggests either an emerged environment or an environment in

transition—subject to periodic tidal inundation as well as freshwater inputs—that the

microfauna (Pozo et al., 2010) as well as the pollen (Zazo et al., 1999a) have indicated.

These data about the ADF substratum and the PHMF formation suggest a landscape in

the estuary right before the first Holocene deposits that consisted of a shoreline

extending far seaward from its present position and made up of a prominent aeolian

system. Research on the sedimentary record in the area of El Abalario (Zazo et al.,

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2005) substantiates this reconstruction further. Toward the east, the ADF would turn

into a muddy alluvial plain (PHMF) which would show clear signs of becoming

terrestrialized (Fig. 7A). Numerous archaeological artifacts found spread on the surface

across El Abalario (Campos et al., 1993; Campos & Gómez-Toscano, 1997) are

evidence of significant cultural activity in the area in the Neolithic and the Copper Age.

The pollen record from the current coasts of southwestern Iberia for the period of

≈10,020±50 to 5380±50 14

C yr BP suggests a wet, temperate climate (Santos et al.,

2003, who failed to calibrate the radiocarbon determinations they adduced). During this

period (the Holocene Climatic Optimum) to judge from the record in the estuaries of

southern Spain (Dabrio et al., 2000) and Portugal (Boski et al., 2001), the level of the

sea rose rapidly at a rate of 5.7 to 8mm/yr. The end of this humid period has been

connected with the Holocene transgressive maximum and, thereby, with the highest

level of area flooded (Zazo et al., 1994). After the Holocene maximum, aeolian dune

systems started to accumulate, particularly from 6,000–5,000 cal. yr BP onward (Zazo

et al., 2008).

5.2. Retro-aggradational Holocene systems

The first morpho-sedimentary units of the Holocene in the Doñana coast took the form

of estuarine barriers and marshland at river mouths in relation to sea level. Analysis of

the cores indicates that the oldest remnants of such Holocene formations found in the

centre of the estuary are those registered in the PN core, dated to 6789-6543 cal. yr BP.

Holocene sedimentation would have spread gradually northward from there to reach S6

and ML ≈6000-5700 cal. yr BP (Fig. 7A). Paleontological evidence suggests a lagoon

environment at the time under considerable marine influence, including the frequent

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effects of high-energy events of sandy sedimentation such as storm surges and the

larger, yet more sporadic, effects of tsunamis. Later, as revealed by overlying deposits

in the cores, a more confined environment became established, increasingly closed to

direct tidal ingress and washover yet open to freshwater inflows from the opposite

direction. This evolution has been recognized elsewhere in the estuary by means of the

paleontological analysis of other cores (Zazo et al., 1999a; Ruiz et al., 2005; Pozo et al.,

2010).

Toward the west (point TC) and south (points CM2 and CM3), in the outer margins of

the estuary, the earliest evidence of Holocene deposits are those of the Spit Formations

(HSF). Made of dunes and beaches that date from 8116-7793 cal. yr BP at a depth of

20 m at TC, such deposits substantiate the formation of a sandy barrier at the beginning

of the Holocene that enclosed part of the estuary and, consequently, enabled

sedimentation therein of finer materials (Fig. 7A). The core at CM2 exposed an

alternation between HSF and HEF, which indicates a punctuated succession over the

past 5000 years of marine–dominated environments (foreshore and nearshore) against

estuarine-dominated environments. At CM3, deposits of two phases of transgressive

dunes (6 to -2 m and -8 to -12 m with respect to present sea level) overlie rather shallow

estuarine formations, in a manner like present-day systems of transverse dunes

encroaching upon the Doñana marshland. On the basis of studies of microfauna,

estuarine-dominated intervals have been interpreted as coastal lagoons evolving into

shallow brackish marshes (Rodríguez-Vidal et al., 2011). Such sedimentary alternation

between shallow estuarine, on the one hand, and foreshore and backshore marine

environments, on the other, fits well with a scenario of retro-aggradational systems

nurtured by progressive subsidence of the ground surface and relative sea-level rise.

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Pollen from the peat of El Acebrón, near the beach resort of Matalascañas, suggests a

dehesa landscape forming in part of the basin c. 5,000 cal. yr BP. Agriculture would

have been practiced there (Stevenson & Harrison, 1992; López-Sáez et al., 2011).

A transitional zone, functioning as a sort of geomorphological threshold, probably under

structural control, would have existed between the outer and inner sides of the estuary.

The Madre de las Marismas fault system, with a NW-SE direction, would have

conditioned a number of blocs and shaped a rising area toward the W and a subsiding

area toward the E (Fig. 7A).

Within the period 4500 to 3500 cal. yr BP, such a transitional zone would have

submerged. Evidence of a tsunami hitting the Gulf of Cadiz c. 4,000 cal. yr BP has been

argued for (Lario et al., 2011), which points to seismic activity in the area at the time.

Turbidite deposits in the Southwestern Portuguese Margin related to a local earthquake

c. 3600 cal. yr BP have been reported as well (Vizcaíno et al., 2006). Because of the

submersion of the transitional zone, sandy deposits encountered in cores S9 and S8 and

dated to 4410-3832 cal. yr BP and 4065-3489 cal. yr BP, respectively, would have

buried and fossilized the existing dune formations of El Abalario (ADF) as under a

large overwash fan introduced into the inner estuary. The exposed geomorphological

evidence of this overwash fan is the littoral strands of Carrizosa-Vetalarena, dated to

4706-4071 cal. yr BP. Clearly laid down by a high-energy event, these marine sands

are evidence of both rupture of the littoral barrier that had formed up to that date and

notable transgression of the sea into the basin (Fig. 7B). The rapidly submerged, new

marine environment can also be inferred from other studies of the area (Rodríguez-

Ramírez et al., 1996; Zazo et al., 1999a; Rodríguez-Ramírez & Yáñez 2008). The

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effects of this event on the Neolithic and Copper Age settlements in the area must have

been considerable, to judge from the numerous archaeological remains found scattered

over the wide range of the Abalario dune systems. The pottery shards from the same

cultural periods encountered embedded in the littoral stands of Carrizosa-Vetalarena are

further indications of the magnitude of the land- and sea-level movement caused by the

event (Fig. 7B).

Eventually, an environment of tidal marshland developed in the estuary, as suggested by

the renewed build-up toward the west of the littoral barrier. Toward the south (points

S8, CM2 and CM3), the thickness of the deposits prevents recognition there of the pre-

Holocene substratum; the sandy bottom at S8 appears to be of the same kind as that

encountered at S9, however.

A number of studies have enabled specialists to estimate for the Gulf of Cadiz that after

the present-day sea level was reached c. 5000 cal. yr BP, the oscillations would not have

exceeded 1m since (Zazo et al., 2008). Yet, as we shall see presently, the identification

at a varying depth of sediments that are elsewhere part of very shallow landforms (e. g.,

exposed prograding units) appears to be at odds with the curve of the mean sea level for

the Gulf presented by Zazo et al. (as cited), which is the generally accepted curve. Such

an anomaly attracts particular attention south of a clear NE-SW structural alignment that

we have called the Torre Carbonero-Marilópez Fault (TCMF). This alignment runs

parallel to both the Guadiamar-Matalascañas Fault (GMF) (Salvany & Custodio, 1995)

and the Lower Guadalquivir Fault (BGF) (Viguier, 1977). North of the TCMF

alignment is the Torre del Loro Fault (TLF), where Zazo et al. (1999b) estimated the

minimum fault throw on the TLF to be 18 to 20m for the Late Pleistocene (Fig. 1). The

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TCMF fault also explains other alignments of lesser morphological expression,

especially south of it, as well as noticeable irregularities in the same direction. The

TCMF alignment acts as a geomorphological as well as a stratigraphic boundary (Fig

2).

Whereas the dune formations of El Abalario, north of the fault, spread conspicuously

over the ground, the corresponding deposits just a few kilometres south of the fault lie

buried at ever lower depths (as much as -20m at the TC point). The fault appears

intersected at intervals by other lines orthogonal to it, following a NW-SE direction; the

most visible are the Madre de las Marismas Fault (MMF) and the Marilópez Fault (MF).

Farther north, the significant geomorphological control affecting the fluvial network

that drains the Neogene formations, with NW-SE and SW-NE directions, bears witness

to the same neotectonic scenario (Flores-Hurtado, 1993) (Fig. 1). Even the coastline

itself can be understood as conditioned by related structural alignments, as suggested by

the cliffs that have developed in Upper Pleistocene formations to the northwest (Goy et

al., 1994; Zazo et al., 1999b). It is commonly accepted that on the coast of the province

of Huelva a number of orthogonal fault systems (following NW-SE, NE-SW and E-W

directions) outline individual blocks that control swamped areas and elevated areas

(Flores-Hurtado, 1993). The received literature (e. g., Zazo et al., 2005; Salvany et al.,

2011) registers no indications that such neotectonics could affect formations that are so

recent, however.

Following the TCMF alignment, the Carrizosa-Vetalarena littoral strands cross the

marshland area to connect with the littoral strand of Marilópez in a chenier system that

has been dated to c. 3800-2500 cal. yr BP (Rodríguez-Ramírez & Yáñez, 2008).

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Located on the elevated structural margin, these strands define a rather clear

geomorphological and palaeogeographic boundary in the marshland area. North of this

boundary lies the domain of old marshland, transformed and degraded; south of it, the

extension of later systems which have gradually replaced a former tidal lagoon (Fig. 8).

Both Carrizosa-Vetalarena and Marilópez outline an ancient shore that remained active

for at least 1300 years, as the subsiding section south of the boundary filled up with

sediments. The infilling proceeded by way of finger-like deltas (Rodríguez-Ramírez et

al., 1996; Rodríguez-Ramírez & Yáñez, 2008) (Fig. 7C, 8).

In short, consistent chronological, stratigraphic and geomorphological data appear to

indicate a tilting of the zone south of the TCMF Fault, especially after c. 4000 cal. yr

BP. Continental formations (ADF) sunk in the process, to be gradually covered by

subsequent estuarine and coastal formations during the course of the Holocene.

Because of such subsidence and the corresponding relative rise in sea level, the littoral

system saw a series of sedimentary sequences of retrogradation and aggradation during

the same time span (Fig. 9).

5.3. Exposed littoral-barrier systems (Progradational systems)

The oldest ages obtained in the exposed beach–barrier systems of the Gulf of Cadiz

come from the Valdelagrana spit, which closes the estuary of the Guadalete River. A

Middle Bronze Age site found within the spit—dated to c. 3800–3600 BP by Gómez-

Ponce et al. (1997) on the typology of some of the archaeological remains encountered;

c. 3500 cal. yr BP at the latest—clearly proves that the Valdelagrana system had already

built up enough to carry a human settlement by 3500 cal. yr BP if not earlier. Indirect

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evidence obtained in other coastal lagoons in SW Spain—e.g., by the spit of Punta

Arenillas in the Odiel-Tinto estuary (Zazo et al., 2008)—suggests that the progradation

of Atlantic beach–barrier systems had started c. 6,500 cal. yr BP.

Although the Guadalquivir estuary is the largest in the Gulf of Cadiz, the exposed

beach–barrier systems in it offering the oldest dates are only those of the littoral strands

of Carrizosa-Vetalarena and Marilópez. As remarked above, these strands are located

inside the estuary and are part of a chenier plain (Rodríguez-Ramírez et al., 1996;

Rodríguez-Ramírez & Yáñez, 2008) (Fig. 1). By contrast, the landforms with the oldest

dates in the spit of Doñana are the littoral strands identified at the LSM location, dated

to 2152-1883 cal. yr BP, as well as the littoral strand of Vetalengua (CV point) which

dates from 2,149-1,825 cal. yr BP. Landforms farther north are made of high piles of

dunes, which house remains of a Roman settlement; this settlement was established by a

hillock known as “Cerro del Trigo” in the 2nd

century CE (Bonsor, 1928; Campos et al.,

2002). In the spit of La Algaida, remains of a pre-Roman settlement from the 6th

to the

2nd

century BC lay buried below the surface (Corzo, 1984) (Fig. 1). Dune systems in

both spits have developed cyclically since, thereby reflecting intense progradation

toward a sea that had reached its present-day level already, or thereabouts (Rodríguez-

Ramírez et al., 1996; Zazo et al., 2008). Analysis of beach ridge morphology, age and

distribution reveals that the prograding systems exposed in the spits record part of

system H5 and the entire system H6. The same analysis provides evidence of periodicity

in relation to climatic oscillation and solar activity fluctuation (Zazo et al., 1994;

Rodríguez-Ramírez et al., 2000, 2003; Goy et al., 2003). Between 2200 and 1200 cal.

yr BP, the formation of a fresh chenier system in the marshland area—namely,

Vetalengua-Las Nuevas—accompanied such prograding developments in the spits,

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ending up in the final infilling of the palaeoestuary and the displacement of the fluvial

network toward the south (Rodríguez-Ramírez & Yáñez, 2008) (Figs. 7D, 9). In such a

1000-year period, the palaeoestuary turned from housing a coastal lagoon—the Lacus

Ligustinus of Roman times—to encompassing a tidal marsh which eventually became a

freshwater marsh. The process made shipping to and from the open sea increasing

difficult, which may well explain why the Cerro del Trigo settlement, a fishing and

salting-industry town, was abandoned in the 6th

century CE.

Whereas indirect data obtained in nearby coastal lagoons of the Gulf of Cadiz (estuaries

of Tinto-Odiel and Guadalete rivers) suggest that the progradation of Atlantic beach–

barrier systems began c. 6500 cal. yr BP (Zazo et al., 1994; Goy et al., 1996; Dabrio et

al., 2000), the said geomorphic and stratigraphic evidence in the Guadalquivir estuary

point to the past two millennia as the period within which net progradation in the

Doñana spit has developed, in connection with a comparatively stable relative sea level

(Fig. 9). Indeed, geomorphic, stratigraphic and chronological analysis of the most

recent phases in the growth of this spit, as well as in the formation of chenier systems in

the estuary, reveals a relative cessation of the sinking processes—at least south of the

TCMF line—in the past 2,000 years or so. Yet between approximately 4000 and 2000

cal. yr BP subsidence was intense there. Relative stability after 2,000 cal. yr BP has

resulted in the growth of littoral and dune systems and in the final infilling of the

palaeoestuary (Fig. 8). The date 2,000 cal. yr BP is thus a turning point between a

retrograding, aggradational system—conditioned by the subsidence-prompted sea-level

rise—and a system that progrades in connection with a relatively stable sea level (Fig.

9). This dramatic change of scenario explains the difference between the Guadalquivir

estuary and other estuaries in the Gulf of Cadiz: in the former, the growth phases of the

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coastal barrier prior to 2,000 cal. yr BP lie buried several metres under the ground

surface.

5.4. Mechanisms for the tectonic activity

It is difficult to determine a single mechanism for the tectonic activity that affects the

Holocene formations in the Guadalquivir estuary. The forces in operation are the

sliding of the Baetic Olistostrome toward the NW and the transpressive tectonics—

together with overpressure (fluid escape)—that generates mud volcanoes and diapiric

processes in this area of the Gulf of Cadiz. Many structures associated with the fluid

escape—including mud volcanoes, mud-carbonate mounds, pockmarks, salt domes and

slides—have been identified and characterized in the platform of the Gulf and put in

connection with NE–SW and NW–SE alignments (Medialdea et al., 2009), in turn

related to the Baetic orogen and the Ibero–African boundary (Maldonado et al., 1999).

Growth faults (normal faults with greater sedimentation on the downthrown side) are a

common geological feature in sedimentary and deltaic basins around the world

(Mauduit & Brun, 1998) and are often located near subsurface reservoirs of

hydrocarbon deposits or salt domes (Jackson et al., 1996), as occurs in the Gulf of

Cadiz. Fluid overpressure within the Pliocene–Pleistocene sedimentary wedge of

coastal plain deposits (Deltaic unit) may have played an important role as well—as it

has a little farther north, in the aeolian formations of El Abalario (Zazo et al., 2005;

Salvany & Custodio, 1995).

Comparable neotectonic scenarios registered elsewhere in the planet (cycles of

subsidence reconstructed for the Cascadian subduction zone (eastern Pacific) and

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Japanese coasts (Bolt, 2003), active fault motion in the Gulf of Mexico (Feagín et al.,

2013; Yeager et al., 2012)), mentioned earlier, provide some illustrative references for

the probable dynamics involved in southern Iberia.

5.5. Archaeological implications

Spanish humanist Antonio de Nebrija, early in the 16th

century CE, was perhaps the first

who called attention to the extent to which the landscape of the Guadalquivir estuary

had changed since Antiquity. A native of the area, Nebrija gave as an example the

disappearance of one of the two mouths of the great river—known as ‘Baetis’ in Roman

times—by the unremitting potency of its alluvial deposits (Nebrija, 1550). In the early

17th

century Juan de Mariana (1852-1853) adduced devastating storms and earthquakes

to also explain such a geologically rapid transformation.

In the late 19th

century historians, linguists and archaeologists began to search the area

for material evidence of human settlement in ancient times. The attempts put to public

view a large number of archaeological sites, some dated to the prehistory and early

history of southern Iberia and others to other periods of history. No doubt the best

known of these initiatives was that of archaeologist G. E. Bonsor and linguist A.

Schulten at the above-mentioned site by Cerro del Trigo, in the 1920s (Bonsor, 1922,

1928; Schulten, 1945), where Roman remains had been unearthed. O. Jessen, the

geologist in the project, found clear evidence of subsidence of the ground surface in the

Doñana spit, but could not explain this natural phenomenon (in Schulten, 1945: 272-

273). No remains of a pre-Roman settlement ever appeared.

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Cerro del Trigo stands on the transgressive dune systems of the Doñana spit (Fig.1). It is

the oldest known settlement on the right-hand side of the Guadalquivir estuary. As

remarked above, it was a fishing and salting-industry town from the 2nd

to the 6th

century CE (Campos et al., 2002). For older settlements, one must search as far as the

currently visible aeolian systems of El Abalario, to the northwest and off the limits of

the spit systems of Doñana (Campos et al., 1993; Campos & Gómez-Toscano, 1997).

The reason for such an archaeological gap of the space in between is, clearly, that pre-

Roman and older formations in the Doñana barrier are several metres below the ground

surface, fossilized by later formations, c. 2000 cal. yr BP old or younger. Possible

anthropogenic remains present in such a space can be found by means of indirect survey

techniques only.

6. Conclusions

Neotectonic activity appears to have conditioned to a significant extent Holocene

sedimentation in the lower Guadalquivir river basin. A set of SW-NE alignments—the

most representative of which being the Torre Carbonero-Marilópez Fault (TCMF)—

interrupted by other alignments following E-W (Marilópez Fault, MF) and NW-SE

(Madre de las Marismas Fault, MMF) directions generate relative movements that can

account for the geomorphological anomalies that are found in the sediments and

landforms of the present.

The oldest Holocene formations on the surface date from 4706-4071 cal. yr BP and are

located north of the TCMF line; south of this fault, by contrast, the visible formations

are c. 2000 cal. yr BP old or younger. This terrain south of the TCMF line appears to

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have tilted. The tilting must have taken place mostly in the years from c. 4000 to 2000

cal. yr BP, when the littoral system developed toward the centre of the basin as a series

of sedimentary sequences of retrogradation and aggradation within a context of sea-

level rise because of the subsidence. In those years the estuary would have been far

more exposed than now to the dynamics of the sea and thereby more prone to high-

energy events such as storm surges and tsunamis and other marine inputs. The strong

subsidence in the centre of the basin explains the apparent interruption in the

archaeological record of the littoral system that affects pre-Roman and older periods in

the Doñana spit. The corresponding Holocene formations underlying the seemingly

missing anthropogenic remains sit several metres below the surface. In the spit of La

Algaida, on the left-hand side of the Guadalquivir estuary, the processes were less

pronounced. From c. 2000 cal. yr BP up to the present, the subsidence has remained

relatively dormant, thereby inviting progradation of the littoral systems and infilling of

the marshland within a context of sea-level stability. During this period the marshland

has been more confined and isolated with respect to marine processes.

In summary, there are good reasons for arguing that the evolution of the Holocene

formations and features in the Guadalquivir estuary, as compared to other estuaries in

the Gulf of Cadiz, has been rather peculiar. Researchers must take this significant

difference into account in attempting to determine regional correlations for southern

Iberia with respect to archaeology, geomorphological and sedimentary processes, sea-

level oscillations, marine inputs (especially tsunamis and storm surges) and climatic

trends during the Holocene.

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The model of evolution presented recalls those of the evolution of zones of similar

geological characteristics in the planet, located near boundaries of tectonic plates, and

can therefore join them as a standard of reference for understanding neotectonic

processes worldwide. We also hope that our integrated study will offer an adequate

geological explanation for the recent geomorphic and sedimentological features

observed in the Doñana UNESCO-MAB Reserve.

Acknowledgements

We are indebted to a vast array of Spanish Institutions: Fundación Caja de Madrid,

Fundación Doñana 21, Ayuntamiento de Hinojos, Fundación FUHEM, Estación

Biológica de Doñana (EBD), Espacio Natural de Doñana (END), Instituto Andaluz del

Patrimonio Histórico (IAPH), Delegación de Cultura of Junta de Andalucía in Huelva

and Organismo Autónomo Parques Nacionales of Ministerio de Medio Ambiente y

Medio Rural y Marino. Without their encouragement and support, the Hinojos Project

would never have sailed. The present paper is a product of the Hinojos Project as well

as a contribution to the IGCP 526 (“Risks, resources, and record of the past on the

continental shelf”), 567 (“Earthquake Archaeology and Palaeoseismology”) and IGCP

588 (“Preparing for coastal change”). We thank reviewers and editors for their helpful

comments. This is publication nº 53 from CEIMAR Publication Series.

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Figure captions

Figure 1.- Study Area. TLF.- Torre del Loro Fault. GMF.- Guadiamar-Matalascañas

Fault. HBF.- Hato Blanco Fault. BGF.- Lower Guadalquivir Fault. TCMF.- Torre

Carbonero-Marilópez Fault. MF.- Marilópez Fault. MMF.- Madre de las Marismas

Fault.

Figure 2.- Correlation of prograding units exposed between the Tinto-Odiel,

Guadalquivir, and Guadalete estuaries. Data from Zazo et al. (1994), Rodríguez-

Ramírez et al. (1996), Goy et al. (1996), Dabrio et al. (2000) and Zazo et al. (2006,

2008).

Figure 3.- Most visible alignments and location of topographic sections (A-B and C-D)

on a 1956 orthophotograph.

Figure 4.- Topographic sections C-D and A-B and abrupt topographic change.

Figure 5.- A relic of old marshland, north of the Carrizosa-Vetalarena sandy ridge.

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Figure 6.- Composition and stratigraphy of the analyzed cores.

Figure 7.- Tectono-sedimentary/geomorphic evolution of the Guadalquivir estuary. A.-

Holocene transgression on the estuary . B.- Activation of TCMF. Strong marine input

on the estuary, subsidence, overwash over ADF, and destruction of human settlements

C.- Sedimentary infilling by fluvial input and evolution from lagoon to tidal marsh. D.

Development of littoral progradational units and sedimentary infilling of estuary to

freshwater marsh.

Figure 8.- Correlation between morphodynamics, sedimentary evolution and human

occupation.

Figure 9.- Lateral and vertical succession of sedimentary bodies in the Guadalquivir

palaeoestuary.

Table captions

Table 1.- Location of the studied cores and elevation with respect to the MSL. (a) UTM

coordinates, Zone 29N. (b) Boreholes drilled by IGME (Geological and Mining Institute

of Spain).

Table 2.- Dates of the different formations. B.- Beta Analytic Laboratory (Miami,

USA). CNA.-Centro Nacional de Aceleradores (Seville, Spain). DAMS.- Accium

BioSciences Accelerator Mass Spectrometry Lab (Seattle, USA). CX.- Geochron

Laboratories, Krueger Enterprises, Inc., (Cambridge, USA). (a)Rodríguez-Ramírez et

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al., 1996. (b)Rodríguez Ramírez & Yáñez, 2008. (

c)Pozo et al., 2010. (

d)Salvany et al.,

2011. (e)Zazo et al., 1999a. (

f)Rodríguez-Vidal et al., (2011). Samples: T.- Transported.

PT.-Poorly transported. NT.-Not transported.

Table 3.- Outline of the sedimentary formations under study.

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Figure 8

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Figure 9

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Table 1

Core X (UTMa) Y(UTM

a) Z (m)

bTC 724,502 4,091,592 +4

bCM2 732,572 4,083,562 +6

bCM3 732,629 4,085,625 +6

bML 737,222 4,100,866 +1.5

bPN 737,503 4,092,111 +2.5

S6 730,636 4,095,665 +2.5

S8 732,979 4,090,362 +1.5

S9 730,173 4,092,611 +2

S11 731,841 4,094,679 +1.5

MA 720,208 4,095,845 +2

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Table 2 Ref. Lab. Core Depth Sample 14C yr (BP) δ13C%o 14C cal. yr 2σ (BP) ΔR (14C yr)

B-287646 S6 -0.5m Cerastoderma edule (PT)

4370±40 -1.8 4071-4706 100±100

B-284999 S6 -10m Glycimeris

sp. (PT)

5300±40 1.4 5693-5910 −135±20

B-285003 S8 -14m Tellina tenuis (NT)

3920±40 1.0 3489-4065 100±100

B-285004 S9 -6m Cerastoderma

edule (NT)

4180±40 0.6 3832-4410 100±100

B-285006 S11 -4m Cerastoderma edule (NT)

3190±40 -2.5 2672-3194 100±100

B-285007 S11 -11.5m Tellina

tenuis(NT)

4860±40 -0.3 5255-5461 −135±20

B-285008 S11 -13m Cerastoderma edule (PT)

19360±80 -25.2 Uncalibrated

DAMS 1217-177 TC -19,8m Venerupis

decussatus (PT)

7349±68 1.6 7793-8116 −135±20

CNA-273 LSM -0,5m Glycimeris

sp. (T)

2260±45 -1.1 1883-2152 −135±20

ªB-88016 CV -0,5m Cerastoderma edule (NT)

2230±60 -- 1825-2149 −135±20

DAMS-1217-178 CM2 -33m Tellina

tenuis(PT)

4820±31 -5.9 5205-5437 −135±20

fB-228874 CM3 -11m shell 2170±40 -- 1805-2046 −135±20

bB-154079 PN -0.5m Cerastoderma

edule (PT)

1960±40 -0.9 1549-1800 −135±20

cB-228881 PN -10.8m shell 4060±40 -- 3665-4269 100±100

cB-228885 PN -14.1m shell 4200±40 -- 3847-4428 100±100

cB-228882 PN -26.1m shell 6090±40 -- 6543-6789 −135±20

d BA PN -35.5m Peat >46,000 -27.5 Uncalibrated

eCX-238339 ML -7.3m shell 3915±50 -1.9 3473-4066 100±100

eCX-23840-A ML -10.8m shell 5370±50 0.4 5728-6012 −135±20

eCX-23841 ML -27m Shell >47,000 -5.8 Uncalibrated

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Table 3 Formations Lithology Mineralogy Paleontology

(macrofossil)

Others Interpretation

PHMF Silts: 60-75%

Clay: 20-25%

Sand: 2-5%

Quartz: 6-10%

Feldspar: 2-4%

Calcite: 15-35%

Dolomite: 0-7%

Absent Lamination

Burrowing, roots

Carbonate nodules

Oxidation

Freshwater to

brackish marsh

ADF Silts: 5-20%

Clay: 1-4%

Sand: 75-90%

Well sorted

Quartz: 75-95%

Feldspar: 5-14%

Filosilicates: 8-

10%

Dolomite: 2-3%

Absent Crossbedding

Oxidation

Bioturbation, roots

Peat bogs

Dune systems

HEF

Clayey Silt

facies:

Silts: 65-70%

Clay: 20-30%

Sand: 0-6%

Quartz: 75-95%

Feldspar: 5-14%

Filosilicates: 8-

10%

Dolomite: 2-3%

Estuarine species

Toward the top,

continental and

freshwater species.

Little or no transport

Burrowing

Lamination

Estuary confined

High fluvial inputs

Tidal to freshwater

marsh

Sandy silt facies:

Silts: 30-40%

Clay: 10-15%

Sand: 10-30%

Quartz: 10-20%

Filosilicates: 30-

50%

Calcite: 15-20%

Estuarine species and

some remains of

transported marine

species

Burrows, roots, and

lamination

Open estuary

Coastal lagoon with

marine input

Sandy layers

facies:

Silts: 25-40%

Clay: 5-10%

Sand: 40-70%

Quartz: 50-95%

Feldspar: 10%

Filosilicates: 10-

40%

Dolomite: 10-15%

Wide diversity of

species of

malacofauna (marine

and estuarine.)

High transport

Coarse boulders

Lithoclasts

Erosive base

Marine input

High energy events

Open estuary

Bioclast layers

facies:

Silts: 40-60%

Clay: 25-30%

Sand: 10-30%

Quartz: 10-20%

Filosilicates: 50-

70%

Calcite: 15-25%

Mostly Cerastoderma

edule and Crassostrea

angulata

Erosive base

Littoral strand

morphology

Cheniers

Dune facies

Sand: 100%.

Well sorted and

rounded

Mean grain size

Quartz: 70-85%

Filosilicates: 15-

25%

Feldspar: 5-10%

Absent Crossbedding Dune systems of the

spit barriers

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HSF

of 125 to 500μm

Beach facies

Fine-to-coarse-

grained sands,

badly sorted.

Quartz: 40-55%

Filosilicates: 10-

20%

Feldspar: 15-25%

Calcite: 10-15%

Wide diversity of

species of marine

malacofauna, rather

deteriorated by

reworking

Lamination,

crossbedding

Beach

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Graphical abstract

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Highlights

New data are provided for the Holocene sedimentary infilling.

New data are provided for the Holocene geomorphic evolution.

Neo-tectonic activity strongly conditions Holocene sedimentation.

The geomorphology and the stratigraphic sequence of the basin are defined.